Optical System

ABSTRACT

An optical system employs a waveguide including a first set of partially-reflecting surfaces (“facets”) for progressively redirecting image illumination propagating from a coupling-in region towards a second region, and a second set of facets in the second region for progressively coupling-out the redirected image illumination towards the eye of a viewer. The first set of facets includes at least a first facet close to the coupling-in region, a third facet fare from the coupling-in region, and a second facet located on a medial plane between the first and the third facets. The second facet is located in a subregion of the medial plane such that image illumination propagating from the coupling-in region to the third facet passes through the medial plane without passing through the second facet.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to optical systems and, in particular, itconcerns an optical system for two-dimensional expansion of an imagefrom an image projector for display to a user.

A near eye display optical engine is shown in FIG. 1A, including animage projector 200 that projects image light having an angular fieldthrough transmissive coupling prism 202T and through vertical aperture203V into waveguide 204. The light propagates in the waveguide, beingreflected by total internal reflection. Partial reflectors 206 embeddedin the waveguide reflect the image out of the waveguide (dashed arrows)towards the observer having eyeball center 208.

FIG. 1B shows an alternative way of coupling into the waveguide by usingreflective coupling prism 202R having mirror on its back side.

FIG. 1C shows schematically a front view of a 2D aperture expansionwaveguide. Here image projector 200 injects an image through couplingprism 202 through lateral aperture 203L (203V is also present, but notvisible from this orientation) into waveguide 204. The image light ray220A propagates laterally in the waveguide as it reflects by TIR betweenthe waveguide faces. Here two sets of facets are used: set 206L expandthe aperture laterally by reflecting the guided image progressively to adifferent guided direction 220B while facets set 206V expand theaperture vertically by progressively coupling the image out from area210 on the waveguide onto the observer's eye.

SUMMARY OF THE INVENTION

The present invention is an optical system for directing imageillumination injected at a coupling-in region to an eye-motion box forviewing by a user.

According to the teachings of an embodiment of the present inventionthere is provided, an optical system for directing image illuminationinjected at a coupling-in region to an eye-motion box for viewing by aneye of a user, the optical system comprising a light-guide opticalelement (LOE) formed from transparent material, the LOE comprising: (a)a first region containing a first set of planar, mutually-parallel,partially-reflecting surfaces having a first orientation; (b) a secondregion containing a second set of planar, mutually-parallel,partially-reflecting surfaces having a second orientation non-parallelto the first orientation; (c) a set of mutually-parallel major externalsurfaces, the major external surfaces extending across the first andsecond regions such that both the first set of partially-reflectingsurfaces and the second set of partially-reflecting surfaces are locatedbetween the major external surfaces, wherein the second set ofpartially-reflecting surfaces are at an oblique angle to the majorexternal surfaces so that a part of image illumination propagatingwithin the LOE by internal reflection at the major external surfacesfrom the first region into the second region is coupled out of the LOEtowards the eye-motion box, and wherein the first set ofpartially-reflecting surfaces are oriented so that a part of imageillumination propagating within the LOE by internal reflection at themajor external surfaces from the coupling-in region is deflected towardsthe second region, wherein the first set of partially-reflectingsurfaces comprises a first partially-reflecting surface proximal to thecoupling-in region so as to contribute to a first part of a field ofview of the user as viewed at the eye-motion box, a thirdpartially-reflecting surface distal to the coupling-in region so as tocontribute to a third part of a field of view of the user as viewed atthe eye-motion box, and a second partially-reflecting surface lying in amedial plane between the first and the third partially-reflectingsurfaces so as to contribute to a second part of a field of view of theuser as viewed at the eye-motion box, wherein the secondpartially-reflecting surface is deployed in a subregion of the medialplane such that image illumination propagating from the coupling-inregion to the third partially-reflecting surface and contributing to thethird part of the field of view of the user as viewed at the eye-motionbox passes through the medial plane without passing through the secondpartially-reflecting surface.

According to a further feature of an embodiment of the presentinvention, the coupling-in region comprises a coupling-in prism having afirst planar surface that is a continuation of one of the major externalsurfaces in the first region, the coupling-in prism having a thicknessdimension measured perpendicular to the major external surfaces that isgreater than a thickness of the LOE.

According to a further feature of an embodiment of the presentinvention, the coupling-in prism presents a coupling-in surface and atransition line between the coupling-in prism as the LOE, thecoupling-in surface defining an optical aperture of the coupling-inprism in a dimension parallel to the major external surfaces and thetransition line defining an optical aperture of the coupling-in prism ina dimension perpendicular to the major external surfaces.

According to a further feature of an embodiment of the presentinvention, the first set of partially-reflecting surfaces furthercomprises at least one partially-reflecting surface located within avolume of the coupling-in prism.

There is also provided according to the teachings of an embodiment ofthe present invention, an optical system for directing imageillumination injected at a coupling-in region to an eye-motion box forviewing by an eye of a user, the optical system comprising a light-guideoptical element (LOE) formed from transparent material, the LOEcomprising: (a) a first region containing a first set of planar,mutually-parallel, partially-reflecting surfaces having a firstorientation; (b) a second region containing a second set of planar,mutually-parallel, partially-reflecting surfaces having a secondorientation non-parallel to the first orientation; (c) a set ofmutually-parallel major external surfaces, the major external surfacesextending across the first and second regions such that both the firstset of partially-reflecting surfaces and the second set ofpartially-reflecting surfaces are located between the major externalsurfaces, wherein the second set of partially-reflecting surfaces are atan oblique angle to the major external surfaces so that a part of imageillumination propagating within the LOE by internal reflection at themajor external surfaces from the first region into the second region iscoupled out of the LOE towards the eye-motion box, and wherein the firstset of partially-reflecting surfaces are oriented so that a part ofimage illumination propagating within the LOE by internal reflection atthe major external surfaces from the coupling-in region is deflectedtowards the second region, wherein the coupling-in region comprises acoupling-in prism having a first planar surface that is a continuationof one of the major external surfaces in the first region, thecoupling-in prism having a thickness dimension measured perpendicular tothe major external surfaces that is greater than a thickness of the LOE,and wherein the coupling-in prism presents a coupling-in surface and atransition line between the coupling-in prism as the LOE, thecoupling-in surface defining an optical aperture of the coupling-inprism in a dimension parallel to the major external surfaces and thetransition line defining an optical aperture of the coupling-in prism ina dimension perpendicular to the major external surfaces.

According to a further feature of an embodiment of the presentinvention, the first set of partially-reflecting surfaces furthercomprises at least one partially-reflecting surface located within avolume of the coupling-in prism.

There is also provided according to the teachings of an embodiment ofthe present invention, an optical system for directing imageillumination injected at a coupling-in region to an eye-motion box forviewing by an eye of a user, the optical system comprising a light-guideoptical element (LOE) formed from transparent material, the LOEcomprising: (a) a first region containing a first set of planar,mutually-parallel, partially-reflecting surfaces having a firstorientation; (b) a second region containing a second set of planar,mutually-parallel, partially-reflecting surfaces having a secondorientation non-parallel to the first orientation; (c) a set ofmutually-parallel major external surfaces, the major external surfacesextending across the first and second regions such that both the firstset of partially-reflecting surfaces and the second set ofpartially-reflecting surfaces are located between the major externalsurfaces, wherein the second set of partially-reflecting surfaces are atan oblique angle to the major external surfaces so that a part of imageillumination propagating within the LOE by internal reflection at themajor external surfaces from the first region into the second region iscoupled out of the LOE towards the eye-motion box, and wherein the firstset of partially-reflecting surfaces are oriented so that a part ofimage illumination propagating within the LOE by internal reflection atthe major external surfaces from the coupling-in region is deflectedtowards the second region, wherein the coupling-in region comprises acoupling-in prism having a first planar surface that is a continuationof one of the major external surfaces in the first region, thecoupling-in prism having a thickness dimension measured perpendicular tothe major external surfaces that is greater than a thickness of the LOE,and wherein the first set of partially-reflecting surfaces furthercomprises at least one partially-reflecting surface located within avolume of the coupling-in prism.

According to a further feature of an embodiment of the presentinvention, the coupling-in prism presents a coupling-in surface and atransition line between the coupling-in prism as the LOE, thecoupling-in surface defining an optical aperture of the coupling-inprism in a dimension parallel to the major external surfaces and thetransition line defining an optical aperture of the coupling-in prism ina dimension perpendicular to the major external surfaces.

According to a further feature of an embodiment of the presentinvention, the coupling-in prism is bonded to the LOE at an edge surfaceof the LOE. Alternatively, the coupling-in prism may be bonded to one ofthe major external surfaces of the LOE.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIGS. 1A and 1B, described above, are schematic side views of aconventional near-eye waveguide-based display illustrating twogeometries for coupling-in of image illumination into the waveguide;

FIG. 1C, described above, is a front view of a conventional near-eyewaveguide-based display illustrating the use of first and second sets ofpartially-reflecting internal surfaces to expand an optical aperture ofan image projector in two dimensions;

FIG. 1D is a schematic isometric view of a waveguide similar to that ofFIG. 1C and corresponding to FIG. 5A of publication WO 2020/049542 A1;

FIGS. 2A-2C are isometric, top and side views, respectively, of anangular representation of the image propagation through an opticalsystem according to the teachings of the present invention;

FIG. 3A is a schematic front view of a light-guide optical element (LOE,or waveguide), constructed and operative according to the teachings ofan aspect of the present invention, illustrating propagation of imageillumination from a coupling-in region to a first set ofpartially-reflecting surfaces (facets), and from the first set of facetsto a second set of facets;

FIG. 3B is a view similar to FIG. 3A illustrating a theoretical locus offacet locations required for providing a field of view (FOV) to a singleviewing point;

FIG. 3C is a view similar to FIG. 3B illustrating a corresponding set ofloci for facet locations to provide a FOV across an “eye-motion box”(EMB) of permitted viewing locations;

FIG. 3D is a view similar to FIG. 3A illustrating the required facetpositions and dimensions in order to span the loci illustrated in FIG.3C;

FIG. 3E is a view similar to FIG. 3C where the loci are furtherincreased according to the geometrical requirements resulting from usingan obliquely-angled first set of facets;

FIG. 3F is a schematic representation of the LOE of FIG. 3D where thefacet-containing regions are demarcated by corresponding polygons, withthe first set of facets demarcated by a concave polygon;

FIG. 3G is a view similar to FIG. 3F in which the concave polygoncontaining the first set of facets is subdivided into a number ofnon-concave blocks or slices;

FIG. 3H is a schematic isometric view illustrating how such blocks canbe assembled to produce the structure of FIG. 3G, for subsequent slicingto form a plurality of LOEs;

FIG. 4A is a view similar to FIG. 3A illustrating exemplary dimensionsfor an implementation generating a rectangular field of view withangular dimensions as illustrated in FIG. 4B;

FIG. 4C is a view similar to FIG. 3A illustrating exemplary dimensionsfor an implementation generating a trapezoidal field of view withangular dimensions as illustrated in FIG. 4D;

FIG. 5A is a view similar to FIG. 4A illustrating an image projector anda coupling-in prism for introducing image illumination into the LOE;

FIG. 5B is a schematic isometric view of the coupling-in prism of FIG.5A;

FIG. 5C is a side view of the coupling-in prism of FIG. 5A illustratingthe required dimensions for an exemplary implementation of the presentinvention;

FIG. 5D illustrates a variant implementation of the present inventionemploying a coupling-in prism which is attached to a major externalsurface of the LOE;

FIG. 6A is a schematic front view similar to FIG. 5A illustrating theuse of an integrated laser-scanning image projector integrated with acoupling-in prism;

FIG. 6B is a schematic side view of the integrated laser-scanning imageprojector integrated with a coupling-in prism of FIG. 6A;

FIG. 7A is a schematic side view of a coupling-in arrangement employingan inclined reflector coupling arrangement and a coupling-in prism, andemploying external collimating optics;

FIG. 7B is a view similar to FIG. 7A in which the inclined reflectorcoupling arrangement and the coupling-in prism are combined and reducedin size;

FIG. 7C is a view similar to FIG. 7A employing a polarized beam splitterand integrating reflective collimating optics into the reflectivecoupling-in arrangement;

FIG. 7D is a view similar to FIG. 7C but using external collimatingoptics;

FIG. 8A is a view similar to FIG. 5A, but where a coupling-in prism isintegrated with part of the waveguide;

FIG. 8B is a schematic isometric view of the coupling-in prism of FIG.8A;

FIG. 9A is a schematic side view of the coupling-in prism of FIG. 8Aimplemented as a coupling-in prism bonded to the LOE at an edge surfaceof the LOE;

FIGS. 9B and 9C are schematic isometric views of the coupling-in prismof FIG. 9A showing inclusion of full or partial partially-reflectivefacets, respectively, within the prism;

FIG. 9D is a schematic side view of the coupling-in prism of FIG. 8Aimplemented as a coupling-in prism bonded to one of the major externalsurfaces of the LOE;

FIGS. 9E and 9F are schematic isometric views of the coupling-in prismof FIG. 9D showing inclusion of full or partial partially-reflectivefacets, respectively, within the prism;

FIG. 10A is a side view of an angular representation similar to FIG. 2Cillustrating two specific points within a field of view of the projectedimage;

FIG. 10B is a partial view similar to FIG. 4A illustrating the imagelight propagation paths corresponding to the two points of FIG. 10A;

FIGS. 10C and 10D are side views of a coupling-in prism indicating theinjection angle of these two field point, respectively, and thecorresponding desired location of the first reflective facet they shouldencounter;

FIGS. 11A-11C are views similar to FIGS. 7A, 7C and 7D, respectively,illustrating implementations of these geometries withpartially-reflective facets within the coupling prisms;

FIG. 11D is a view similar to FIG. 6B illustrating an implementation ofthis geometry with partially-reflective facets within the couplingprism;

FIG. 12A is a schematic isometric view of a series of plates withselectively-deployed partially-reflecting coatings, each according to apattern required for a different plane of the LOE of FIG. 3D, forassembly according to a production method of the present invention;

FIG. 12B is a schematic isometric view of a stack of plates formed bybonding together the series of plates of FIG. 12A;

FIG. 12C is a schematic isometric view of a block formed by slicing thestack of FIG. 12B along the indicated dashed lines;

FIG. 12D is a schematic isometric view of (a part of) an LOE formed byslicing the block of FIG. 12C along the indicated dashed lines;

FIG. 13 is a schematic side view of a coupling-in configurationemploying an air gap and a mirror surface;

FIGS. 14A and 14B are views similar to FIGS. 2A and 2C, respectively,showing the image propagation through the optical system in the case ofan obliquely oriented first set of partially-reflecting surfaces;

FIG. 14C is a view similar to FIG. 3F for an implementation of the LOEoptimized for the image propagation described in FIGS. 14A and 14B;

FIG. 15 is a schematic side view of a side coupling-in configurationemploying a beam-splitter to fill the waveguide with the injected imageand its conjugate;

FIG. 16A is a side view of an angular representation of the imagepropagation through the second region of an LOE according to a variantimplementation of the present invention employing a high-inclinationsecond set of partially-reflecting surfaces;

FIG. 16B is a schematic side view of the second region of the LOEimplemented according to the optical geometry of FIG. 16A;

FIG. 16C is a graph indicating schematically the preferred angledependence of reflectance of the facets for the implementation of FIG.16B;

FIG. 17A is a schematic isometric view of a plate withselectively-deployed partially-reflecting coatings, similar to theplates of FIG. 12A;

FIG. 17B is an enlarged schematic side view illustrating a coated regionimplemented with an abrupt edge;

FIG. 17C is a schematic illustration of the use of a raised mask togenerate a coating with a marginal region of gradually-varyingthickness;

FIG. 17D is a schematic illustration of the use of the principle of FIG.17C to deposit a multi-layer coating with a marginal region ofgradually-varying thickness;

FIG. 17E is a schematic illustration of the use of a second raised maskto generate a complementary transparent coating in regions not coated bythe first process; and

FIG. 17F is a schematic side view illustrating a partially-reflectingregion resulting from the sequence of coating processes described withreference to FIGS. 17D and 17E.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is an optical system for directing imageillumination injected at a coupling-in region to an eye-motion box forviewing by a user.

By way of introduction, in the context of near-eye displays of the sortillustrated in FIG. 1C, it has been found that particularly advantageousgeometrical properties, and in particular, minimization of thedimensions required for a given angular field of view, may be providedby injecting image illumination into the waveguide at “shallow angles”,meaning that all of the image illumination is incident on only one sideof both the first and the second sets of facets. Typically, in shallowangle implementations, at least part of each image lies within about 15degrees, and more preferably within about 10 degrees, from the plane ofthe external surfaces of the waveguide. This results in a shortenedlight path from the image projector to the observer's eye, and hencealso enables reduced size of the optical components for a given angularfield of view. The present invention relates to a number of aspectswhich facilitate shallow-angle implementations of such a display, whichpresents certain design challenges, particularly with regard tocoupling-in configurations. It should be noted, however, that variousaspects of the invention described herein are not limited toshallow-angle implementations, and may also be applicable to otherimplementations.

FIGS. 2A-2C show an angular polar representation of an image as itpropagates in the waveguide according to present invention. FIG. 2Ashows an isometric view, FIG. 2C a side view and FIG. 2B shows a topview (relative to FIG. 2A) that corresponds to the view from the frontof the waveguide.

The waveguide has total-internal-reflection (TIR) boundary circles 228,indicating that images within these circles are not subject to TIR, andwill be coupled-out so as to escape the waveguide.

Image 220A1 is coupled into the waveguide and propagates by TIR back andforth to 220A2. These images propagate along a very shallow trajectoryalong the waveguide where the shallowest part of the image is only 7degrees from the waveguide plane (shown as angle 221 in FIG. 2C). Facets224 (equivalent to 206L of FIG. 1C) in this implementation areperpendicular to the waveguide, and therefore reflect images 220A1 and220A2 directly onto 220B1 and 220B2, respectively. Images 220B1 and220B2 are coupled by TIR as they propagate down the waveguide. In thesecond portion of the LOE, facets 226 (equivalent to 206V in FIG. 1C)couple image 220B2 out of the waveguide onto image 220C towards theobserver.

By way of one non-limiting example, the illustrations shown hereinrelate primarily to an image having aspect of 4:3 and diagonal field of70 degrees injected into a waveguide having refractive index on 1.6. Thedesign illustrated here generates a full image at an eyeball center 35mm away from the waveguide (including eye-relief, eyeball-radius andmargins). Adaptations of these implementations for different fields ofview and aspect ratios can readily be implemented by a person ordinarilyskilled in the art on the basis of the description herein.

One aspect of the present invention relates to optimization ofdeployment of partially-reflecting surfaces (or “facets”) in the firstpart of the waveguide responsible for the first dimension of opticalaperture expansion. In an earlier patent application published as WO2020/049542 A1 (“the '542 publication”), it has been suggested to deployfacets selectively within an envelope encompassing the facets which areneeded for delivering image illumination to the eye-motion box fromwhich the image is to be viewed.

An example of the resulting deployment of facets is illustrated in FIG.5A of that publication, which is reproduced here as FIG. 1D. In thatimplementation, an optical system for directing image illuminationinjected at a coupling-in region 15 to an eye-motion box 26 for viewingby an eye of a user employs a light-guide optical element (LOE) 12formed from transparent material having a first region 16 containing afirst set of planar, mutually-parallel, partially-reflecting surfaces 17having a first orientation, and a second region 18 containing a secondset of planar, mutually-parallel, partially-reflecting surfaces 19having a second orientation non-parallel to the first orientation. A setof mutually-parallel major external surfaces 24 extend across the firstand second regions such that both the first set of partially-reflectingsurfaces and the second set of partially-reflecting surfaces are locatedbetween the major external surfaces. The second set ofpartially-reflecting surfaces 19 are at an oblique angle to the majorexternal surfaces 24 so that a part of image illumination propagatingwithin the LOE by internal reflection at the major external surfacesfrom the first region into the second region is coupled out of the LOEfrom a coupling-out region 28 towards the eye-motion box 26. The firstset of partially-reflecting surfaces 17 are oriented so that a part ofimage illumination propagating within the LOE by internal reflection atthe major external surfaces from the coupling-in region is deflectedtowards the second region.

According to the teachings of the '542 publication, certain parts of thefirst region 16 of the LOE outside the envelope of useful facets areimplemented as an optical continuum (i.e., without partially reflectinginternal surfaces), thereby reducing unwanted “ghost” reflections.Within the convex polygonal envelope, however, the facets areimplemented as filling the entire width of the convex polygon, asillustrated in the drawing. As a result, the part of the field of viewreflected from the facets located on the side distal from thecoupling-in region pass through a long series of partially-reflectingfacets before reaching the facets which deliver that part of the fieldof view to the eye-motion box.

According to one aspect of the present invention, the regions of facetsrequired to deliver a given field of view to the eye-motion box isfurther refined to generate a concave polygon defining the requiredfacet locations, thereby removing parts of the intermediate facets whichwould otherwise unnecessarily attenuate the image illumination directedto provide the part of the field reflected by facets furthest from thecoupling-in region.

Thus, as illustrated in FIG. 3D, the first set of partially-reflectingsurfaces, here labeled 206L, includes a first partially-reflectingsurface 17A proximal to the coupling-in region 240 so as to contributeto a first part of a field of view (FOV) of the user as viewed at theeye-motion box, a third partially-reflecting surface 17C distal to thecoupling-in region so as to contribute to a third part of the FOV of theuser as viewed at the eye-motion box, and a second partially-reflectingsurface 17B lying in a medial plane 22 between the first and the thirdpartially-reflecting surfaces so as to contribute to a second part ofthe FOV of the user as viewed at the eye-motion box. In the exampleillustrated in FIG. 3D, facet 17A contributes to the right side of theFOV, facet 17C contributes to the left side of the FOV, and facet 17Bcontributes to the central region of the FOV. It is a particular featureof this aspect of the present invention that the secondpartially-reflecting surface 17B is deployed in a subregion of themedial plane 22 such that image illumination propagating from thecoupling-in region to the third partially-reflecting surface (arrow 23)and contributing to the third part of the field of view of the user asviewed at the eye-motion box passes through the medial plane 22 withoutpassing through the second partially-reflecting surface 17B.

In this context, it should be noted that the terms “proximal”, “distal”and “medial” are used herein to denote relative position with respect toa point or region of interest, in this case the coupling-in region 240,and refer to facets which are relatively closer (proximal) to, orrelatively further (distal) from, the coupling-in region, or which are“towards the middle” (medial), without necessarily denoting the closest,furthest or central facet according to any specific geometricaldefinition.

A conceptual explanation will now be provided in order to facilitate abetter understanding of the geometrical optics considerations which leadto the preferred design parameters for a given implementation of thisaspect of the present invention. It should be noted that thisexplanation is given for informational purposes only, but that theutility of the invention as claimed is not dependent on the accuracy ofany aspect of this explanation, and that effective and advantageousimplementations of the claimed invention may alternatively beimplemented by empirical methods.

FIG. 3A shows front view of few selected beams of the projected imagehaving parameters corresponding to the exemplary FOV mentioned above.Solid lines represent beam of an image injected into the waveguidelaterally and dashed lines represent beams after lateral apertureexpansion and reflection as they propagate vertically. It should benoted throughout this document that any example describing lateralexpansion followed by vertical expansion can be changed to verticalexpansion followed by lateral without inherent change in structure. Thiscan be exemplified simply by rotating the above figures by 90 degrees.

All beams are transmitted from entrance pupil of the coupling-in region240. The beams propagate within the waveguide until being reflected byset of parallel embedded reflectors 206L (facets). The facets in thisdiagram are assumed to be perpendicular to the external faces of thewaveguide. Therefore, every line (solid followed by dashed) represents adifferent lateral section of the projected image field onto theobserver's eye. The vertical field of every section is illuminated byplurality of overlapping beams (as viewed from front) propagating atdifferent angles inclination (into the page) that are reflected by TIR(such internal reflection being illustrated in the side view of FIG.1A).

For the purpose of simplifying the geometrical analysis, assume firstthat only the eyeball center 208 needs to be illuminated with the entirelateral field. This can theoretically be achieved by having infinitelysmall lateral facts 206L placed infinitely close to each other along thetrajectory represented as 244A in FIG. 3B. However, the requirement toaccommodate lateral shifting of the eyeball center (for example due tovariation of interpupillary distance between users) dictates a shift ofcurve 244A. FIG. 3C shows schematically the curve for three lateralpositions of the eyeball 208 as 244A, 244B and 244C. To cover therequired width of the eye-motion box, widening of the facets isrequired.

Other requirements include:

There is a finite minimum distance between the facets (i.e., they cannotbe infinitely close); The aperture size cannot be too small; The facetsmust project continuous reflections towards the vertical expansionfacets 206V.

FIG. 3D shows the finite size facets 206L appropriate for the aboveconditions. The length of the facets can vary according to the facetspacing and other optical parameters such as refractive index, the sizeof the projected field and the location of image injection 240.

In the case where the lateral expanding facets 206L are at an obliqueangle to the major external surfaces of the LOE, the image is injectedinto the waveguide rotated relative to the axes of the waveguide, andthe reflection from facets 206L rotates the image to the requiredorientation. Consequently, the curve for projecting onto the center ofeyeball 208 will look like FIG. 3C, and when additionally consideringthe spread required for the lateral eye-motion box, it will look likeFIG. 3E, that shows further multiplication of curves 244A, 244B and244C. This therefore requires somewhat wider facets than a correspondingimplementation with orthogonal facets for the first expansion.

Certain advantages of this aspect of the present invention can be betterappreciated with reference to FIG. 3F. The area of the lateral expandingfacets is described at SL while the area of the ‘depression’ above it isSD and the vertical expanding facets area is SV. Due to the concavepolygon form of SL, the light propagating laterally from the entrance240, propagates mostly in transparent area SD before being reflecteddownward in SL. The propagation in a transparent area reduces the lossof image illumination by reflection to undesired directions, therebyimproving waveguide efficiency. Furthermore, there are no undesiredreflections of the scenery by facets from SD, thereby substantiallyreducing glints and ghost images from the waveguide.

One possible method for producing the lateral expansion section SL with‘depression’ SD is shown in FIGS. 3G and 3H. The sections including SDand SL are subdivided into blocks as indicated by heavy outlines in theregion designate 300 in FIG. 3G. FIG. 3H illustrates how this structurecan be assembled from a transparent prism 302 having appropriate faceangles together with three plates having appropriate facet angles (shownas lines along the plates) that fit together against the correspondingsurfaces of prism 302 and against each other to form the assembledstructure as illustrated. This structure is combined with additionalclear prisms and the vertical-expansion portion of the LOE to generatethe overall structure.

Optionally, the combined prism and plates can be sliced, or can first beattached to another stack to be sliced together to generate thewaveguide with all its sections.

The size of the waveguide as described above is shown in FIG. 4A,resulting in an image field angular size as shown in FIG. 4B. It isnoted that the most laterally-spread beams 250 illuminate only the lowercorners of the image, thereby requiring a large waveguide area whilecontributing only to a small part of the image. In certain applications,it may be acceptable, or even advantageous, to provide a non-rectangularFOV, and in particular, a trapezoid image field, as illustrated in FIG.4D. In this case, there is shown an image which has the same total areaas FIG. 4B (which is shown with dashed lines in FIG. 4D for comparison),but distributed as a trapezoid, with a wider field at the top than atthe bottom. The LOE to generate this FOV is illustrated in FIG. 4C, withcorresponding dimensions. In this case, lateral edge light beams 252illuminate vertically all the field (with corresponding different anglesinto the paper), therefore making much more efficient use of the LOEsize. Consequently, the size of the waveguide of FIG. 4C issubstantially smaller than that of FIG. 4A, as illustrated by theexemplary dimensions for the same overall FOV area. The subsequentdescription with illustrate further aspects of the invention in thenon-limiting exemplary context of the configuration of FIG. 4A, but itshould be appreciated that configurations such as that of FIG. 4C may beimplemented using the same principles.

Injecting a shallow image into a waveguide requires a relatively largecoupling prism 202. FIG. 5A shows the waveguide with lateral entrancepupil 203L and coupling prism 202M to scale. The image projector 200 isillustrated schematically. FIG. 5B is an isometric view of the couplingprism 202M with vertical aperture 203T and lateral aperture 203L, bothlocated at same plane so as to define a rectangular aperture. FIG. 5Cillustrates a side view of coupling prism 202M which, for a 1.7 mm thickwaveguide, requires a 14 mm-long coupling prism designed to couple lightfrom all field angles into the waveguide. Part of the beams 262 arereflected from the bottom of the prism before entering through pupil203V into waveguide 204. The height of prism 202M above the waveguide is6.4 mm, which is acceptable in many applications. However, the size ofprojector 200 that is need to inject the image through prism 202M alsotakes space and volume and, in many applications, will not beacceptable.

The prism 202M (and others described herein) preferably has a lower facethat is parallel to the waveguide faces for consistent reflection, whilethe upper and side faces do not need specific optical properties, sotheir shapes can be other than the ones shown in these figures.

FIG. 5D illustrates an alternative architecture where a coupling prism202L is located above the waveguide, and where the entrance pupil is theprism face 264. In this case, the prism and the required projector arelarger than in FIGS. 5A-5C, making this configuration non-optimal.

One option for reducing the overall dimensions of the couplingarrangement and image projector is illustrated in FIGS. 6A and 6B, whichillustrate integration of the image projector with the coupling prism. Ascanning laser image projector is illustrated here by way of anon-limiting example, but the same principles can be implemented usingan image projector based on another type of image generator, such asemploying an LCOS (liquid crystal on silicon) spatial light modulator,or a micro-LED image generator.

FIG. 6A shows a coupling prism 202P attached to waveguide 204. FIG. 6Bshows a side view of the integrated (embedded) image projector. Laser270 directs a polarized beam onto scanning mirrors 272 that scanintermediate image plane across a micro-lens array (MLA) or diffuser274. The scanned light passes through the diffuser and is reflected frompolarizing-beam-splitter 276 onto collimating reflecting lens 278(combined with a quarter-wave plate). The reflected light passes throughPBS 276 into coupling prism 202P. This coupling prism thus serves alsoas part of the PBS 276. Part of the light passes directly into thewaveguide and part is reflected by the lower face 279 before enteringthe waveguide, thereby filling the aperture of the waveguide with boththe image and its conjugate.

Parenthetically, wherever a PBS arrangement is illustrated herein assequentially reflecting and then transmitting light, or the reverse, itwill be understood that a half-wave plate (for single transmission) or aquarter-wave plate (for double transmission) is appropriately placed toachieve rotation of the polarization as required for the functionalitydescribed. The polarization-rotating elements will not be mentioned ineach case.

Alternative architectures for combining the image projector with thecoupling prism are shown in FIGS. 7A-7D. FIG. 7A shows light from imagegenerator (not shown, but as before, may be a scanned laser, LCOS orother) being collimated by a refractive lens 280 (beams in diagram fromdifferent fields points therefore not parallel), entering prism section282 and being reflected by mirror 284 into prism section 202Q and intowaveguide 204. In this configuration, prism sections 282 and 202Q can becombined to a single prism, as illustrated in FIG. 7B. Furthermore,there is no need for the light to be polarized in this case.

The structure of FIG. 7B employs an equivalent reflector architecture toFIG. 7A, but with smaller prism. Here the prism length is of the orderof 14 mm, similar to prism 202M in FIG. 5C for similar outputparameters. However, the height will be only 3.2 mm, which is half thatof 202M. As in all of the prisms described herein, the upper(non-reflecting) face of the prism 283 is preferably absorbing. It isdrawn here according to the upper beam 260 (defined in FIG. 5C), butsince the upper surface is not optically significant, it can be higherand/or have other shapes.

This prism can also be provided with a coupling configuration includinga lower refractive index part at its lower face equivalent to elements286 or 228 described below with reference to FIGS. 7C and 7D. Theinterface can be used to attached to a PBS as an image projector.

FIG. 7C shows injection of diverging polarized light corresponding to animage (originated from a MLA, scanning laser, LCOS or other imagegenerator) passing through interface 286 into PBS section 290 andreflected by PBS 292 onto reflecting collimating lens 294. The reflectedcollimated light passes through PBS 292 into coupling prism 202Q andinto the waveguide 204. In this architecture some of the light isreflected by the lower section of prisms 290 and 202Q, therefore thesesurfaces need to have good image quality and to be continuous. Theinterface 286 can be air, but also a low refractive index medium(relative to prism 290), so that TIR will occur for light reflected from294.

FIG. 7D shows an arrangement similar to FIG. 7C, but with externaloptics (e.g., a refractive lens 296) that collimates the light and aflat reflector 298.

Turning now to FIGS. 8A and 8B, these illustrate an alternativeconfiguration according to a further aspect of the present invention inwhich a coupling prism is integrated with part of the waveguide insteadof as an extension shown in FIG. 5A. FIG. 8A shows coupling prism 202Y(dotted area) on top of waveguide 204. Image generator 200 is thereforelocated closer to the waveguide and has a smaller size. All of the lightrays propagating in the waveguide continue to emerge from point 240,therefore the lateral aperture 203L2 is located at same place as theprevious embodiments. However, the vertical aperture 203V is herelocated at the end of the prism, where the thickened portion of theprism meets with the major external surface which defines the mainportion of the LOE. FIG. 8B shows the shape of the coupling prism in anisometric view. The two apertures 203L2 and 203V2 have same width aspreviously described but because of the separation of location the prismbecomes vertically elongated at 203L2.

It is apparent from FIG. 8A that some of the facets (here represented as206A and 206B) are located within the prism. A number of possibleimplementations of these facets in prism 202Y are illustrated in FIGS.9A-9F. For clarity of presentation, the facets lying within the mainpart of the LOE have been omitted here.

FIG. 9A shows prism 202Y attached to the edge of the waveguide, and FIG.9B shows in isometry the placement of the facets in the prism. However,since there is no need for the facets to reflect light across the entirewidth of the prism (as can be seen in FIG. 8A, which illustrates thatthe required width of facets 206A and 206B is limited), FIG. 9C showsthe reflecting parts (shaded area) to be only a part of thecorresponding plane within the prism.

FIG. 9D shows an alternative configuration where prism 202Y is formed byattaching a correspondingly-shaped block on top of waveguide 204 (facetsin 204 are not shown). In FIG. 9E, the same structure is illustratedwith facets across the entire cross-section of 202Y above the LOEthickness, while in FIG. 9F, the reflective area is implemented only asthe shaded region, corresponding to only the optimal required area.

In certain cases, it is possible to combine the earlier-mentionedintegrated image projector (FIGS. 6A-7D) together with theLOE-integrated coupling prism of FIGS. 8A-9F. Certain geometricalconsiderations in such an implementation are illustrated with referenceto FIGS. 10A-10D.

FIG. 10A shows a side view of an angular distribution equivalent to FIG.2C, but with markings of two field points associated with facets 206A(circle) and 206B (square). Same field points are shown in FIG. 10B.

FIG. 10C shows a side view of the waveguide-overlapping coupling prismwhere the shaded area represents a preferred location for the facetsassociated with 206A and FIG. 10D shows the preferred location for thefacets for 206B. The PBS 292 is shown here for reference. It is apparentthat the preferred location for the facets in the overlapping couplingprism should be above the PBS plane. Implementation of facets at beforethe PBS plane would cause distortion to the transmitted image.

FIGS. 11A-11D are side views illustrating implementations of facets tocoupling prisms incorporating projector optics. FIGS. 11A, 11B and 11Cshow integration of facet section 202T (marked as a shaded area) intoconfigurations which are otherwise similar to those of FIGS. 7A, 7C and7D, respectively. The 3D representation remains as was described withreference to 10A-10D.

FIG. 11D parallels FIG. 6B, and illustrates that, where the PBSorientation is opposite, the facet section 202T2 is best implementedonly partially after the PBS plane. The coupling prism will thereforeextend slightly further outside lateral aperture plane 203L2(illustrated in FIGS. 8A-8B).

Parenthetically, the deployment of facets 202T2 as illustrated in FIG.11D will also be suitable for configurations employing the waveguidearchitecture of FIGS. 4C and 4D.

Turning now to FIGS. 12A-12D, as an alternative to the productionprocess illustrated above with reference to FIGS. 3G-3H, the facetpatterns of FIG. 3D or 3F can be produced based on stacking and slicingselectively-coated plates. In this case, the plates are coated in apredefined pattern as shown in FIG. 12A, which shows a set of platesshown from the front 300F that are coated in predefined patterns 302F.These patters have width and position according to required coatedfacets shown in 112. These patterns are preferably produced by maskingthe uncoated part of the waveguide while coating. It is also possible tocoat only the other part of the face of the plates in 304F by coating anon-reflective coating in order to maintain flat surface or to preservethe phase of transmitted light through 304 to be equivalent to phase oftransmitted light through 302.

FIG. 12B illustrates a stack formed by bonding together the partiallycoated plates, where the dashed line shows the slicing planes across thestack. FIG. 12C shows one slice having side view of the plates 300S andthe reflective patterns 302S. Another slice is done as shown by thedashed lines in FIG. 12C to generate the final upper section of FIG.12D, corresponding to the upper part of the LOE of FIG. 3D.

It should be noted that the order of the slicing may be changed. It willalso be appreciated that the illustrations of FIGS. 12A-12D are highlyschematic, and that a larger number of plates are typically used.

Reducing the refractive index of the coupling prism 202M can also beused for reducing prism size and thereby making the system more compact.FIG. 13 illustrates an extreme case of this concept where 202M isreplaced with air-gap and mirror 310 that is in-plane with lowerwaveguide plane 312. The light from projecting optics 308 is directedonto perpendicular entrance 306 to waveguide 204 and onto mirror plane310. Because of the lower refractive index of the air-gap, angles of thebeams change and consequently the length of the mirror is shorter thanlength of prism 202M. The angle of the lower beam 262 is now 11.5degrees instead of 7 before. Consequently, the mirror length is now 8.5mm instead of 14 mm in FIG. 5C. As the beam enter the waveguide itsangular distribution is as in FIG. 5C. The mirror can overlap thewaveguide for mechanical attachment.

A conceptually-similar approach of employing low refractive indexmaterial can be implemented using a low refractive index glass prism.When using a low refractive index glass prism, it is possible tocompensate some of the dispersion generated by the angle in incidence ofthe light entering face 306.

In the embodiments detailed thus far, the facets employed 206L employedfor the first set of facets have been orthogonal to the major externalsurfaces of the LOE, as detailed in FIGS. 2A-2C. In an alternative setof embodiments illustrated with reference to FIGS. 14A-14C, the firstset of facets is implemented using obliquely angle facets 336. FIG. 14Ashows an isometric angular representation of such system used totransmit shallow angle images, while FIG. 14B shows a correspondingpartial side view of the angular representation. This non-limitingexample employs a trapezoidal FOV, equivalent to the image for minimalsize shown in FIGS. 4C and 4D, although this structure could clearlyalso be used for a rectangular FOV.

In this architecture the initial laterally propagating image 334A1 iscoupled with 334A2 by TIR reflections. Only 334A2 is redirected towardsthe second region of the LOE 334B1 by tilted facets 336, which are at anoblique angle relative to waveguide faces. Facets 336 are preferablycoated with multilayer dielectric coatings, as is known in the art, toprovide the desired degree of partial reflectivity for the range ofincident angles corresponding to image 334A2 (as in all of the aboveembodiments), while being primarily transparent to the range of incidentangles corresponding to image 334A1, so as to minimize energy losses andformation of undesired reflections. The image 334B1 is coupled to 334B1by TIR as it propagates at shallow angle along the second region of thewaveguide. Image 334B2 is then coupled out to 334C by facets 338 (shownonly in FIG. 14A), in a manner equivalent to facets 226 of FIGS. 2A-2C.

FIG. 14C illustrates the waveguide footprint that is equivalent to FIG.4C, where shaded region 340 is the optimal area for the first set offacets 336, and area 342 is the optimal area for output coupling facets338.

The various coupling-in prism arrangements described above areconfigured to couple-in to the LOE both an image and its conjugate to“fill” the waveguide with the image. An alternative approachparticularly attractive for shallow-angle image injection into thewaveguide is direct injection of image into the waveguide, asillustrated in FIG. 15. In order to fill waveguide 370 with the injectedimage and its conjugate, waveguide 370 has a coupling region 372 thathas a partial reflector 374 along a center plane of the waveguide. Thepartial reflector 374 is most preferably implemented as an 50%reflector, preferably insensitive to angle and achromatic, such as apartially-silvered surface.

In FIG. 15, three beams are shown associated with the lowest point inthe field, and are therefore the shallowest beams of the imageillumination. The lower beam (solid arrow) passes through collimatingoptics 376 and a coupling prism 378 and enters the waveguide. After onereflection, it experiences partial reflection by 374 and is split intotwo beams. The central beam (dashed line) is split at the entrance andthe top beam (dash-dot line) is split half way along combiner 372. It isapparent that after the beams are split (thereby splitting the imageillumination between the image and its conjugate), the waveguide isilluminated uniformly. Therefore, uniform image is expected after lightis coupled out.

If the partial reflector 374 has 50% reflectivity and 50% transmittancethen for length equivalent to that of FIG. 5C (14 mm in our example),the waveguide will be illuminated uniformly. In this configuration theaperture of the illuminating optics 376 is very small since the opticsis almost adjacent to the entrance of the waveguide, resulting in asmall thickness of the optical assembly.

Turning now to FIGS. 16A-16C, this illustrates an alternative scheme forcoupling-out of the image in the second region of the LOE toward theeye-motion box for viewing by the eye of the user. The angularrepresentation of FIG. 16A is similar to FIG. 2C, but in this case, theoutput coupling facets 390 have a steep angle. As a result, image 220B1is coupled out to 220C (instead of 220B1 as in FIG. 2C). In thisconfiguration, the images 220B can be taller and are not limited by theangle of the facets 390.

FIG. 16B shows schematically how such a configuration looks in realspace. As the beam propagates downward (in this drawing), it ispartially reflected by the facets downward out of the waveguide. In sucha configuration it is preferable to have the facets closely spaced inorder to ensure a uniform image.

FIG. 16C shows schematically the preferred reflectivity of facets forsuch configuration. Here, low reflectivity is desired at the lowincidence angles (close to perpendicular) and higher reflectivity (foroutput coupling) at higher angles. Here too, such properties are readilyachieved using appropriately designed multilayer dielectric coatings, asis well-known in the art.

Turning now to FIGS. 17A-17F, various of the preferred embodimentsdescribed herein require partial selective application of reflectivecoatings on only part of a plate which is then assembled into a stackfrom which part or all of the LOE is then sliced. Partial coating offacets as illustrated in FIG. 17A could potentially introduce scatteringeffects at the edges of the coating, due to the physical discontinuityin coating as illustrated in schematic cross section in FIG. 17B.Additionally, this mechanical discontinuity may cause mechanical stresson the plates when stacked (as in FIG. 12B). FIGS. 17C-17F illustrate apreferred production method according to an aspect of the presentinvention for overcoming these limitations.

FIG. 17C illustrates the principles of coating characteristics when amask 394 is placed close to plate surface 300F but slightly spaced fromthe surface. When implementing coating (thick arrow), it will coat theplate where there is no mask but close to the mask a gradual decree incoating thickness will be generated around the edge of the mask 396.FIG. 17D illustrates schematically how this characteristic can be usedto generate a gradual decrease (“tail-off”) of the coating pattern 302Fat the periphery of the desired region.

For thin coating thickness, this configuration of gradual thinning ofthe coating may be sufficient. For thicker coatings, it may beadvantageous to use a second mask over the region 302F (FIG. 17E) and toapply a complimentary transparent coating 98 beside the reflecting area302F, as illustrated in FIG. 17F. It is noted that the mask of FIG. 17Eis typically not the exact inverse of the mask of FIG. 17D, since it ispreferably increased around the boundary by an amount corresponding tothe tailing-off region (which can be determined empirically).

It will be appreciated that the above descriptions are intended only toserve as examples, and that many other embodiments are possible withinthe scope of the present invention as defined in the appended claims.

What is claimed is:
 1. An optical system for directing imageillumination injected at a coupling-in region to an eye-motion box forviewing by an eye of a user, the optical system comprising a light-guideoptical element (LOE) formed from transparent material, said LOEcomprising: (a) a first region containing a first set of planar,mutually-parallel, partially-reflecting surfaces having a firstorientation; (b) a second region containing a second set of planar,mutually-parallel, partially-reflecting surfaces having a secondorientation non-parallel to said first orientation; (c) a set ofmutually-parallel major external surfaces, said major external surfacesextending across said first and second regions such that both said firstset of partially-reflecting surfaces and said second set ofpartially-reflecting surfaces are located between said major externalsurfaces, wherein said second set of partially-reflecting surfaces areat an oblique angle to said major external surfaces so that a part ofimage illumination propagating within said LOE by internal reflection atsaid major external surfaces from said first region into said secondregion is coupled out of said LOE towards the eye-motion box, andwherein said first set of partially-reflecting surfaces are oriented sothat a part of image illumination propagating within said LOE byinternal reflection at said major external surfaces from saidcoupling-in region is deflected towards said second region, wherein saidfirst set of partially-reflecting surfaces comprises a firstpartially-reflecting surface proximal to the coupling-in region so as tocontribute to a first part of a field of view of the user as viewed atthe eye-motion box, a third partially-reflecting surface distal to thecoupling-in region so as to contribute to a third part of a field ofview of the user as viewed at the eye-motion box, and a secondpartially-reflecting surface lying in a medial plane between said firstand said third partially-reflecting surfaces so as to contribute to asecond part of a field of view of the user as viewed at the eye-motionbox, wherein said second partially-reflecting surface is deployed in asubregion of said medial plane such that image illumination propagatingfrom said coupling-in region to said third partially-reflecting surfaceand contributing to the third part of the field of view of the user asviewed at the eye-motion box passes through said medial plane withoutpassing through said second partially-reflecting surface.
 2. The opticalsystem of claim 1, wherein said coupling-in region comprises acoupling-in prism having a first planar surface that is a continuationof one of said major external surfaces in said first region, saidcoupling-in prism having a thickness dimension measured perpendicular tosaid major external surfaces that is greater than a thickness of saidLOE.
 3. The optical system of claim 2, wherein said coupling-in prismpresents a coupling-in surface and a transition line between saidcoupling-in prism as said LOE, said coupling-in surface defining anoptical aperture of said coupling-in prism in a dimension parallel tosaid major external surfaces and said transition line defining anoptical aperture of said coupling-in prism in a dimension perpendicularto said major external surfaces.
 4. The optical system of claim 2,wherein said first set of partially-reflecting surfaces furthercomprises at least one partially-reflecting surface located within avolume of said coupling-in prism.
 5. An optical system for directingimage illumination injected at a coupling-in region to an eye-motion boxfor viewing by an eye of a user, the optical system comprising alight-guide optical element (LOE) formed from transparent material, saidLOE comprising: (a) a first region containing a first set of planar,mutually-parallel, partially-reflecting surfaces having a firstorientation; (b) a second region containing a second set of planar,mutually-parallel, partially-reflecting surfaces having a secondorientation non-parallel to said first orientation; (c) a set ofmutually-parallel major external surfaces, said major external surfacesextending across said first and second regions such that both said firstset of partially-reflecting surfaces and said second set ofpartially-reflecting surfaces are located between said major externalsurfaces, wherein said second set of partially-reflecting surfaces areat an oblique angle to said major external surfaces so that a part ofimage illumination propagating within said LOE by internal reflection atsaid major external surfaces from said first region into said secondregion is coupled out of said LOE towards the eye-motion box, andwherein said first set of partially-reflecting surfaces are oriented sothat a part of image illumination propagating within said LOE byinternal reflection at said major external surfaces from saidcoupling-in region is deflected towards said second region, wherein saidcoupling-in region comprises a coupling-in prism having a first planarsurface that is a continuation of one of said major external surfaces insaid first region, said coupling-in prism having a thickness dimensionmeasured perpendicular to said major external surfaces that is greaterthan a thickness of said LOE, and wherein said coupling-in prismpresents a coupling-in surface and a transition line between saidcoupling-in prism as said LOE, said coupling-in surface defining anoptical aperture of said coupling-in prism in a dimension parallel tosaid major external surfaces and said transition line defining anoptical aperture of said coupling-in prism in a dimension perpendicularto said major external surfaces.
 6. The optical system of claim 5,wherein said first set of partially-reflecting surfaces furthercomprises at least one partially-reflecting surface located within avolume of said coupling-in prism.
 7. An optical system for directingimage illumination injected at a coupling-in region to an eye-motion boxfor viewing by an eye of a user, the optical system comprising alight-guide optical element (LOE) formed from transparent material, saidLOE comprising: (a) a first region containing a first set of planar,mutually-parallel, partially-reflecting surfaces having a firstorientation; (b) a second region containing a second set of planar,mutually-parallel, partially-reflecting surfaces having a secondorientation non-parallel to said first orientation; (c) a set ofmutually-parallel major external surfaces, said major external surfacesextending across said first and second regions such that both said firstset of partially-reflecting surfaces and said second set ofpartially-reflecting surfaces are located between said major externalsurfaces, wherein said second set of partially-reflecting surfaces areat an oblique angle to said major external surfaces so that a part ofimage illumination propagating within said LOE by internal reflection atsaid major external surfaces from said first region into said secondregion is coupled out of said LOE towards the eye-motion box, andwherein said first set of partially-reflecting surfaces are oriented sothat a part of image illumination propagating within said LOE byinternal reflection at said major external surfaces from saidcoupling-in region is deflected towards said second region, wherein saidcoupling-in region comprises a coupling-in prism having a first planarsurface that is a continuation of one of said major external surfaces insaid first region, said coupling-in prism having a thickness dimensionmeasured perpendicular to said major external surfaces that is greaterthan a thickness of said LOE, and wherein said first set ofpartially-reflecting surfaces further comprises at least onepartially-reflecting surface located within a volume of said coupling-inprism.
 8. The optical system of claim 7, wherein said coupling-in prismpresents a coupling-in surface and a transition line between saidcoupling-in prism as said LOE, said coupling-in surface defining anoptical aperture of said coupling-in prism in a dimension parallel tosaid major external surfaces and said transition line defining anoptical aperture of said coupling-in prism in a dimension perpendicularto said major external surfaces.
 9. The optical system of claim 2,wherein said coupling-in prism is bonded to said LOE at an edge surfaceof said LOE.
 10. The optical system of claim 2, wherein said coupling-inprism is bonded to one of said major external surfaces of said LOE. 11.The optical system of claim 3, wherein said coupling-in prism is bondedto said LOE at an edge surface of said LOE.
 12. The optical system ofclaim 3, wherein said coupling-in prism is bonded to one of said majorexternal surfaces of said LOE.
 13. The optical system of claim 4,wherein said coupling-in prism is bonded to said LOE at an edge surfaceof said LOE.
 14. The optical system of claim 4, wherein said coupling-inprism is bonded to one of said major external surfaces of said LOE 15.The optical system of claim 5, wherein said coupling-in prism is bondedto said LOE at an edge surface of said LOE.
 16. The optical system ofclaim 5, wherein said coupling-in prism is bonded to one of said majorexternal surfaces of said LOE.
 17. The optical system of claim 6,wherein said coupling-in prism is bonded to said LOE at an edge surfaceof said LOE.
 18. The optical system of claim 6, wherein said coupling-inprism is bonded to one of said major external surfaces of said LOE. 19.The optical system of claim 7, wherein said coupling-in prism is bondedto said LOE at an edge surface of said LOE.
 20. The optical system ofclaim 7, wherein said coupling-in prism is bonded to one of said majorexternal surfaces of said LOE.